Public Switched Telephone Network (PSTN) gateways must reliably transport a variety of signals over Internet Protocol (IP) networks. Previously, people have focused on transporting fax and modem signals. However, there is also a need to reliably transport signals emitted from PSTN textphone, or TTY/TDD, devices used by the deaf and hard of hearing.
PSTN gateways, alternatively referred as Voice Over IP (VoIP) gateways, compress audio signals from a PSTN network and convert the compressed audio signals into packets that are then transported over an IP network. Voice compression is effective when processing speech signals, but introduces problems when processing and transmitting PSTN signals representing text. In addition, the IP network may occasionally drop packets. This packet loss may be acceptable for voice data but may not provide acceptable character error rates for text transport.
A wide range of applications employ techniques for transporting text over IP, including e-mail, instant messaging, Internet Relay Chat, newsgroups, and Internet Engineering Task Force (IETF) Request For Comments (RFC) 2793 (“RTP Payload for Text Conversation”). Most of those applications are either not real-time (e.g., e-mail) or are quasi-real-time (e.g., Instant Messaging). Generally, the Instant Messaging applications and others in that category do not transport text character-by-character.
The Request for Comment (RFC) 2793 specifies a way of transporting text in real-time between two devices over a Real Time Transport Protocol (RTP) stream, but transports audio and text independently on separate RTP streams. Using dual RTP transport streams increases management complexity in the gateway and limits the number of audio/text calls that can be processed.
The present invention addresses this and other problems associated with the prior art.